Trends in aerosol optical depth in northern China retrieved from sunshine duration data

A new method has been developed to retrieve aerosol optical depth (AOD) from sunshine duration (SSD). Retrieved AODs from SSD at the six stations in northern China in 2003-2005 agree reasonably well with AODs retrieved from Moderate Resolution Imaging Spectroradiometer observations near the six stations. Values and trends in AOD retrieved from SSD in Beijing and Tianjin in the period 1961-2005 also agree with those retrieved from solar radiation and visibility. These agreements allow the retrieval of credible upper and lower limits for anthropogenic AODs from SSD at the six stations during 1961-2005. The trends in anthropogenic AODs are approximately a factor of 3 to 5 lower than the trends in emissions of gas-phase precursors of aerosols in 1973-2005, implying a significant sublinear relationship between the level of aerosols and emissions of their gas phase precursors. This finding has important implications for formulating a control strategy for PM2.5 or haze pollution in northern China.